This invention relates to a method and apparatus for high temperature heating, melting, refining and superheating of materials, such as steel scrap, metals, ceramics or glass. The method and apparatus disclosed may be used as the major source of energy and also as an assisting energy source in melting furnaces, industrial heating and heat treating furnaces, kilns, incinerators and other high temperature applications.
Today, scrap preheating and melting is accomplished by different technologies, such as heat from the combustion of coke, oil or gas with air or oxygen, or from electric arc. Each of these technologies has some advantages and disadvantages. Using air for combustion has the advantage of being a low cost oxidizer, but, because air only contains 21% oxygen, it has the following disadvantages: low flame temperature, combustion instability inside cold scrap, low efficiency of heat utilization when scrap is hot due to escaping flue gases which waste about 50% of heat released by combustion.
The advantages of using essentially pure oxygen for combustion include: high flame temperature, good combustion stability, and a significant reduction of wasting heat with hot flue gases. The disadvantages of oxygen include its high cost and the necessity to cool the oxygen-fuel burner body. The utilization of electrical energy is very expensive, but it provides a convenient means of operation and high product quality.
Burners for combusting fuel with air are old in the art, and burners for combusting fuel with pure oxygen (oxy-fuel) are well known. Furthermore, oxygen enriched air has been used as the oxidizer in burners. However, the current state of the art burners do not operate satisfactorily across the full range of temperatures useful in high temperature heating, and do not allow for economical operation through control of flame chemistry, temperature, velocity and luminosity. Burners designed for use with hot air or oxygen enriched air typically use refractory tiles in the burner for continuous igniting of gases to stabilize the flame. However, due to the very high temperature of an oxy-fuel flame, refractory tiles cannot be used, and such burners are internally water or air cooled. The elimination of the burner tile results in flame instability at lower temperatures and therefore limits the turn-down ratio of oxygen enriched air burners.
Another problem that often arises in oxy-fuel and oxygen enriched burners is the presence of excess oxygen in the flue gases. The hot furnace temperatures, together with the excess oxidizing ability of the flue gases, accelerate deterioration of expensive furnace components.
Also, in cases where natural gas is utilized as a fuel, an oxygen-fuel flame or an oxygen enriched air-fuel flame is not emissive. To be able to transfer heat, the flame would therefore have to touch the product being heated. This can create a problem with product distortion and oxidation. Improved emissivity of natural gas flames is a priority concern of this invention.